A contact sheet is put between electronic devices, such as integrated circuits, cables, and printed circuit boards, and is used to connect terminals of electric devices in order to shorten the length of electrical path and to reduce the resistance between them. Such a contact sheet is composed of conductive contacts that are arranged on edge part of a package or in a grid pattern on a base sheet. Generally, each contact is fixed on the base sheet. The portion of the contact that contacts the terminal is elasticized and extends a distance above or below the base sheet and elastically contacts the terminal of the electronic device to make an electrical connection.
In recent years, because of the demand for high-density pin-arrangements and installing a large number of pins, the lead pitch in electronic devices is becoming increasingly finer. Even in these cases, it is required that the contacts provide a stable electronic connection with respect to displacement and load. In accordance with this requirement, the size of the contact has been reduced so far.
However, according to this former way, the shorter the length of the contact is, the smaller the displacement length becomes, and it is impossible to absorb any warp caused during contact with the electronic device. If, however, the contact length is preserved by instead reducing the width or the thickness of the contact (in order to maintain sufficient displacement length), a sufficient contact load between the terminal and the contact cannot be ensured.
For the above-mentioned reasons, a contact having a spiral structure was developed. Both ends of the contact are positioned between base sheets and supported by therein, and a part of the contact that connects to the terminal is formed as a spiral in the vertical direction of the base sheet (U.S. Pat. No. 5,297,967). Although the contact sheet according to U.S. Pat. No. 5,297,967 can absorb the warp of the electronic device due to the sufficient displacement length of its contact connecting part, the resistance increases, however, and the contact load tends to be small. As a result, the electrical resistance becomes larger as a whole and it is difficult to decrease the circuit inductance and increase the operation speed.
Furthermore, a structure was developed wherein both ends of the contact are positioned between base sheets and an intermediate part of the contact is twisted, extending from the upper and lower surface of the base sheets, and formed into a connecting part (U.S. Pat. No. 6,328,573 B1). In the case of the contact sheet according to U.S. Pat. No. 6,328,573 B1, however, even if the connecting part is twisted, the electrical resistance is increased along with the displacement length.